The purpose of this project is to study physiological and pathological aspects of the renin-angiotensin system, with emphasis on its role on circulatory homeostasis and development. Previous studies of this laboratory have demonstrated the expression of widely distributed angiotensin II (AII) receptors in the fetus. Experiments using newly developed AII receptor subtype specific antagonists showed that while AII receptors at most sites in the adult are type 1 (AT1), the majority of the receptors transiently expressed during early development are type 2 (AT2). An interesting finding was that in two major AII target tissues in the adult, the adrenal glomerulosa and vascular smooth muscle, the concentration of AII receptors in markedly higher during early postnatal development, due to the presence of high AT2 levels. The ratio AT1/AT2 receptor content in the adrenal glomerulosa was 0.25, 0.96, 2.4 and 5.6 for 2-, 7-, 14, 21-, and 40-day old rats, respectively). In vivo experiments showed that the sensitivity of the plasma aldosterone responses to AII injection in 7-day old rats is lower than in adults. The possibility that the low adrenal sensitivity to AII in neonates is due to the high proportion of AT2 receptors was studied in collagenase dispersed adrenal glomerulosa cells from 7-day old rats. Despite the high AT2 receptor content (30%), the AT1 receptor antagonist completely inhibited AII-stimulated aldosterone production, and the AT2 receptor antagonist had no effect. The data show that AT2 receptors do not have a modulatory role on the steroidogenic effects of AII, but more likely participate in adrenal growth. Supporting the later possibility was the finding that sodium restriction, condition associated to hypertrophy of the adrenal glomerulosa also causes an increase in adrenal AT2 receptor content. Polymerase chain reaction (PCR) techniques were employed to obtain specific cDNA probes for the brain AII receptor. Restriction analysis of the PCR products and preliminary hybridization studies using different probes show heterogeneity in the sequence of the brain and peripheral receptors. Studies in cultured fetal skin fibroblasts are also in progress to determine the participation of AII receptor subtypes on the expression of growth factors and constitutional cell proteins such as Beta-actin.